Area: 0.71 hectares.
A new site, listed in the Geological Conservation Review as of national importance.
Description and Reasons for Notification:
This site comprises a small quarry and tor exposures in quartz-topaz- tourmaline rocks at the southern edge of the Permo-Carboniferous St. Austell granite. Modal variation has resulted in the rocks being banded in places and tourmaline-topaz and quartz-rich varieties are present.
Although close to the margin of the granite the rocks do not represent a sheeted roof complex, such as that seen at Porthcew, as their composition is too refractory to have separated directly from a magma. It is rather thought that the St. Mewan rocks represent the crystallised silica-rich fraction of a hydrothermal saline ore-bearing fluid, which became entrapped beneath the impermeable pelitic roof rock at St. Mewan.
This site is of great scientific interest as it documents an important evolutionary stage between late-stage granite processes and later mineralising events in south-west England.